1. Field of the Invention
This present invention relates to a wireless communications system for the communication of content data, the system being of the type used to support Wireless Local Area Networks (WLANs), for example, a cellular communications system. The present invention also relates to a method of communicating content data in the wireless communications system.
2. Background of the Invention
A cellular communications network usually comprises one or more Base Station Transceivers (BTS). Mobile and fixed subscriber stations located in the cellular communications network communicate with the one or more BTS, for example, a closest BTS. Each of the one or more BTS has a limited range corresponding to a geographical region over which the BTS is intended to operate effectively, the geographic region being known as a “cell”. The geographic region can be further supported in regions of poor radio coverage and/or high demand by sub-cells having a radius in the region of 100-400 meters and known as micro-cells. Where smaller areas of coverage are required, for example inside buildings, the sub-cells can have a radius in the region of 50-100 meters, known as pico-cells. The micro-, or pico-cells can provide overlapping coverage for the geographical region. In an in-building communications system, for example a WLAN, the mobile and fixed subscriber stations (hereinafter referred to as the terminals) communicate with WLAN BTSs. Typically, there are more WLAN base stations per terminal than BTSs per terminal in the cellular communication networks, because the WLAN base stations have respectively smaller coverage areas and there is a need to compensate for blocked or reduced signal propagation caused by internal walls of the buildings. Indeed, the internal walls act as signal reflectors.
In cellular communications systems, including WLANs, each terminal is arranged to communicate with a single base station; in the case described above the single base station is the closest BTS. Each terminal communicating with a given BTS requires a certain amount of bandwidth to operate, a total amount of the bandwidth of the given BTS being limited. Consequently, a total number of terminals capable of communicating with the given BTS is limited.
Additionally, increases in bandwidth requirements, either through an increase in the total number of terminals needing to communicate with the given BTS, or through needs of individual terminals to support more sophisticated or better services, for example high data rates, can be dealt with over a long time frame by, for example, installing new BTSs and splitting cells. Although the number of cells increases, the provision of additional BTSs or splitting cells is expensive, requires a geographical site, and is time consuming in deployment. Disadvantageously, if geographical distribution of the bandwidth requirements changes over time so that the bandwidth requirements for the given BTS reduces, any newly installed BTSs will become redundant. In the case of the WLAN, redeployment of staff within a building/group of buildings is an example of the change of bandwidth requirements.
Given that an aim of wireless communications system design is to reduce the number of BTSs required by increasing the range of the BTSs and/or capacity, i.e. the number of terminals able to communicate effectively with a given antenna arrangement, there is a tendency for cellular communications network operators to consider alternative ways of increasing capacity in the cellular communications network. In the case of data communications, traffic generated by the terminals is in the form of bursts of traffic, known as bursty traffic. Consequently, there is a requirement for an instantaneous high bandwidth of, typically, short duration. However, of the total number of terminals capable of communication with the given BTS, only a fraction of the total number of terminals are simultaneously active at a given time.
In the example of the WLAN, all of the terminals may be switched on and logged onto the WLAN, but only 10% of the terminals may actually be actively using the WLAN and communicating content, for example voice or data traffic. Of the 10% of the total number of the terminals, only a small proportion thereof may be actively downloading data at any instant in time. In this respect, the European Telecommunications Standards Institute (ETSI) 3rd Generation (3G) data model recommendations for simulation purposes are that Web browsers download new web pages at about 4 minute intervals, and the average download volume is of the order of 20 kbytes. Thus, only a small number of terminals can be expected to be downloading at any particular instant in time.
Code Division Multiple Access (CDMA) schemes are known to employ “soft handoff” as a way of improving capacity of cellular communications systems. A mobile terminal at an edge of a cell is typically capable of communicating with two, or three BTSs capable of transmitting signals at similar signal strength levels from centres of nearby cells. If the mobile terminal is only communicating with one BTS, a Carrier-to-Interference (C/I) ratio is usually low due to the mobile terminal being at the edge of the cell. Consequently, a Quality of Service (QoS) provided by the mobile terminal is often poor and it is not uncommon for error rates to be high and calls to be dropped.
If, however, more than one BTS is communicating with the mobile terminal and all the BTSs communicating with the mobile terminal are controlled to transmit exactly the same data waveforms to the mobile terminal, then receivers in the mobile terminal process the same data from a single BTS. Consequently, the C/I ratio is increased by a factor of two or three.